The present invention relates to a continuous process for preparing lactic acid from an aqueous feed which has been obtained by fermentation and mainly contains lactate. The term xe2x80x9ca feed which mainly contains lactatexe2x80x9d refers to a feed which contains about 250 g of lactate ion/l or less, based on the total amount of the feed, at least 90 wt % of the lactate ion being present as free lactate ion and the remaining quantity of the lactate ion being present as undissociated acid.
Many methods are known from the prior art. WO 99/19290, published on Apr. 22, 1999, describes various methods for purifying lactic acid from an aqueous composition of lactic acid and a salt of lactic acid (a lactate) dissolved therein, the aqueous composition having been obtained by fermentation of a suitable feed at a pH of 4.8 or less (claim 1). The pH of the aqueous composition is therefore moderately to strongly acidic, so that the composition contains a substantial amount of lactic acid and a substantial amount of lactate. According to page 4, lines 21 to 28 of WO 99/19290, this is actually advantageous, since the lactate after separation of lactic acid can be recycled to the fermentation step where it serves as a buffering agent for controlling the pH during the fermentation. On the other hand, page 14, lines 21 to 30 of WO 99/19290 states that the methods can be implemented using compositions having a pH of at least 0.86 to less than 6.0, which implies that at 25xc2x0 C., the molar ratio of lactic acid and its salt in these compositions is between 1000: 1 and 0.007:1. Preferably, these compositions have a pH of from 1.98 to 5.00 (molar ratio of lactic acid and its salt is between 75:1 and 0.070:1) and in particular a pH of from 3.0 to 4.5 (molar ratio of lactic acid and its salt is between 7.0:1 and 0.23:1). The lactic acid obtained by the method according to WO 99/19290 is used, in particular, for preparing polylactic acid.
Page 40, line 23, to page 41, line 11, of WO 99/19290 describes a preferred embodiment, wherein a suitable culture medium is fermented to form a composition which contains lactic acid and a lactate. In a subsequent step, the solids are separated by means of, for example, filtration, flocculation, centrifuging or a combination of such techniques, to form an aqueous mixture of lactic acid and lactate. The aqueous mixture of lactic acid and lactate is passed directly to an extraction apparatus, although it is possible for the aqueous mixture to undergo an operation before being passed to the extraction apparatus. Such an operation, for example, is an adjustment of the pH by the addition of an acid such as sulphuric acid, lactate thereby being converted into lactic acid (p. 5, lines 8 to 14 of WO 99/19290). Such an operation would have the drawback, however, of leading to the formation of a waste product consisting, for example, of calcium sulphate (calcium sulphate is formed by the reaction of the lactate formed during the fermentation with sulphuric acid; in the course of the fermentation, for example in the start-up phase, calcium carbonate is then added to maintain the pH of the fermentation medium at a sufficiently high value). Moreover, p. 13, lines 33-36 of WO 99/19290 clearly states that the methods described in this reference are especially suitable for purifying lactic acid from relatively acidic culture media which need not be treated with an acid and which preferably have indeed not undergone such an operation.
According to the above-described preferred embodiment of WO 99/19290, the aqueous mixture of lactic acid and lactate is separated, by means of extraction, into an organic phase which mainly contains lactic acid and an aqueous phase which mainly contains lactate. This extraction can be carried out at a temperature of from 15xc2x0 C. to 60xc2x0 C. and atmospheric pressure, and according to p. 28, lines 29 to 32 of WO 99/19290, the organic extractant used comprises a tertiary amine, an oxygen-containing solvent and a hydrocarbon, preferably from 60 to 80% by weight of a tertiary alkylamine such as Alamine 336 (a mixture of trialkylamines having at least 18 carbon atoms, the trialkylamines containing decyl and octyl groups) which is marketed by Henkel Corp., from 5 to 20% by weight of methyl isobutyl ketone and from 10 to 30% by weight of a hydrocarbon such as Isopar K(trademark). Then the organic phase which mainly contains lactic acid is subjected to a back extraction, the lactic acid again being transferred into a polar solvent phase, for example water. This back extraction takes place, for example, at a temperature of from 30xc2x0 C. to 160xc2x0 C. or more, and customarily at a temperature of from 90xc2x0 C. to 160xc2x0 C. (page 30 lines 26 to 28 of WO 99/19290). According to page 26, lines 22 and 23 of WO 99/19290, this back extraction can also be carried out at a temperature of at least 100xc2x0 C., for example 150xc2x0 C. or more, and at a pressure of at least 30 psi(g) (about 2.1 bar). The polar solvent phase which is obtained from the back extraction and which contains lactic acid is then concentrated by means of distillation, either the lactic acid or the polar solvent being separated as a vapour, this being dependent on the respective volatilities. P. 20, lines 15 to 21 of WO 99/19290, describes that such a distillation is preferably carried out at a pressure of less than 300 mmHg (about 0.4 bar), to prevent as far as possible the dimerization and oligomerization of lactic acid. The aqueous phase which is formed in the extraction and mainly contains lactate is recycled (optionally after purification) to the fermentation step. For this method to be feasible and effective, the aqueous mixture of lactic acid and lactate which is purified according to this method must necessarily contain a substantial amount of lactic acid and a substantial amount of lactate. It is therefore necessary for the aqueous mixture of the lactic acid and lactate to be moderately to strongly acidic, for example 4.8 or less. The pKa of lactic acid is about 3.86 at 25xc2x0 C., and about 3.89 at 50xc2x0 C.; see also FIG. 11 of WO 99/19290).
The above-described preferred embodiment of the method for purifying lactic acid has a number of drawbacks. For the purpose of large-scale purification or continuous purification, the extraction should be carried out under carbon dioxide pressure, as described in claim 30 of the U.S. Pat. No. 5,510,526, wherein the partial carbon dioxide pressure is at least 50 psi(g) (about 3.5 bar), relatively expensive equipment being required consequentially which is able to withstand a relatively high pressure. Furthermore, the kinetics of the extraction of the carbon dioxide pressure proves sluggish. It was found that for effective extraction, the contact between the organic phase and the water phase has to be maintained for about 100 hours to reach equilibrium in a stirred system.
Yet another drawback of the abovementioned preferred embodiment is that recycling of the mainly lactate-containing water phase to the fermentation results in accumulation of lactate and impurities in the fermentation. No mass balances are described in WO 99/19290, but it is necessary for part of the aqueous composition of lactic acid and lactate formed during the fermentation to be removed from the fermentation (via a so-called sink or xe2x80x9cpurgexe2x80x9d), which is not described in WO 99/19290, however. The need for a sink or xe2x80x9cpurgexe2x80x9d with a recycling step of this type is described, in contrast, in WO 98/15517 on page 5, last paragraph. WO 98/15517 and WO 99/19290, moreover, come from the same applicant and list the same inventors. The use of a sink or xe2x80x9cpurgexe2x80x9d naturally reduces the yield of the purification. According to the preferred embodiment the throughput is therefore low, and the preferred embodiment has a relatively low yield per unit weight of feed per unit time. The lack of experimental data in WO 99/19290 moreover makes it very difficult for those skilled in the art to reproduce the method described therein.
The extraction under carbon dioxide pressure also gives rise to calcium carbonate. In practice it was found that this calcium carbonate is relatively impure and is short of mineral nutrients and therefore cannot be recycled to the fermentation. This extraction step according to the above-described preferred embodiment therefore gives rise to a waste product, a problem that WO 99/19290 is precisely attempting to prevent. Page 5, lines 3 to 18, describes that the first step of a customary purification of an aqueous composition of lactic acid and lactate comprises the addition of sulphuric acid, calcium sulphate being formed in the process (if calcium carbonate has been used as a pH regulator during the fermentation). Said calcium sulphate is stated to be a waste product. However, it was found that the costs associated with the removal of the waste product calcium sulphate and the procurement of sulphuric acid are considerably lower than the costs associated with the use of carbon dioxide during the extraction step. A further drawback of this method is that a three- or sometimes even a four-phase system is formed (organic phase, water phase, solid phase in the form of calcium carbonate and a gas phase in the form of carbon dioxide), therefore requiring complex and consequently expensive equipment.
Moreover, it was found that in the case of extraction under carbon dioxide pressure, the mainly lactate-containing aqueous phase contains relatively large amounts of lactic acid, so that efficient purification of lactic acid from the aqueous composition obtained from the fermentation requires a recycling step of said aqueous phase. The yield of this preferred embodiment (i.e. (1) fermentation, (2) extraction, (3) back extraction and (4) concentration) of WO 99/19290 is therefore relatively low and provides lactic acid of inadequate purity. Furthermore, said preferred embodiment is either difficult to implement or not sufficiently clearly described in WO 99/19290, especially in view of the lack of experimental data. A further drawback of this method is that it involves a crystallization process (calcium carbonate has to precipitate for the equilibrium to be shifted sufficiently far towards lactic acid), which makes the method highly malfunction-prone. Moreover, the presence of the solid calcium carbonate can easily lead to the formation of emulsions.
WO 98/37050, published on Aug. 27, 1998, describes a method of producing lactic acid and products thereof from a medium which contains an alkaline earth metal lactate, wherein a conjugated base of the alkali metal, obtained from a step carried out previously, is reacted with the medium to form a water-soluble alkali metal lactate and a basic compound of the alkaline earth metal, the separation of the water-soluble alkali metal lactate and the basic compound of the alkaline earth metal, and cleaving the water-soluble alkali metal lactate to form a conjugated base of the alkali metal and a lactic acid product. The lactic acid product can be lactic acid as such, a derivative or a combination of lactic acid and the derivative.
WO 98/15519, published on Apr. 16, 1998, describes a method of recovering purified lactic acid products from an aqueous feed which contains lactic acid, lactate or a mixture thereof, wherein the feed is brought into contact with a substantially immiscible anion exchanger to form a substantially water-immiscible phase which comprises an adduct of the ion exchanger and lactic acid, effecting a condensation reaction in the substantially water-immiscible phase between a carboxyl group of lactic acid and a hydroxyl group, a primary amine group or a secondary amine group to form a lactic acid ester or a lactic acid amide, and the separation of the lactic acid ester or the lactic acid amide from the ion exchanger.
WO 98/15517, published on Apr. 16, 1998, describes a method of recovering lactic acid and products thereof from an aqueous solution which contains lactic acid and a lactate. The method comprises the extraction of at least 70% of lactic acid from the aqueous solution by bringing the solution into contact with a basic extractant to form a lactic acid-containing extract and an aqueous lactate solution, the separation of the lactic acid-containing extract and the aqueous lactate solution, and stripping the lactic acid-containing extract by means of methods known per se. According to a preferred embodiment, the aqueous lactate solution can be acidified, for example with sulphuric acid to form lactic acid and calcium sulphate (waste product), and the lactic acid can then be removed from the aqueous solution, for example by extraction. The extraction step according to WO 98/15517 likewise has to be carried out under carbon dioxide pressure. According to Example 8, this pressure can be 30 atmospheres (about 30 bar). The method according to WO 98/15517 therefore has the drawbacks of calcium carbonate (formed during the extraction carried out under carbon dioxide pressure) and calcium sulphate being formed as waste products, high costs and a relatively low yield and a relatively low throughput.
U.S. Pat. No. 5,766,439, granted on Jun. 16, 1998, describes a method for producing an organic acid such as lactic acid, fermentation giving rise to an aqueous mixture of an organic mono-, di- or tribasic acid having from three to eight carbon atoms. The mixture is admixed with an alkaline earth metal base in such an amount that the pH of the mixture is sufficiently high, a salt of the acid and the alkaline earth metal being formed. The salt of the acid and the alkaline earth metal is then reacted with ammonium ions to form an ammonium salt of the acid. The ammonium salt is then converted into the free acid by means of so-called salt-splitting electrodialysis.
WO 96/01247, published on Jan. 18, 1996, describes a method for recovering a carboxylic acid from an amine-based, water-immiscible solution of an extract which contains an extraction-enhancing agent. An impure aqueous solution of the carboxylic acid is brought into contact with the solution of the extractant, after which the extraction-enhancing agent is extracted from the carboxylic acid-containing extract obtained, by means of an aqueous solution of a salt of the carboxylic acid to form a water phase which contains extraction-enhancing agent and a carboxylic acid-containing extract (organic phase). From the water phase which contains extraction-enhancing agent, the extraction-enhancing agent is removed by means of distillation. The carboxylic acid is recovered from the acid-containing extract by means of back extraction with water. Lactic acid is not mentioned in WO 96/01247.
WO 98/55442 describes a method wherein a solution of lactic acid in water as obtained from fermentation or another source is subjected to at least three steps. The first step comprises the removal of such ionogenic substances from the aqueous solution of lactic acid as are able to catalyse the oligomerization of lactic acid, the solution containing less than 80%, preferably less than 50% and in particular less than 30% of lactic acid. Preferably, an anion exchanger is used to remove cationogenic substances, and a cation exchanger is then used to remove anionogenic substances. The second step comprises concentrating the solution to a concentration of from 50 to 90%, preferably from 70 to 90%, by evaporation at reduced pressure, the pressure being from 50 to 500 mbar and preferably from 50 to 250 mbar and the temperature being kept as low as possible. Preferably, the evaporation is carried out with the aid of flowing-film evaporation. The third step comprises a distillation at a pressure of from 0.001 to 100 mbar, preferably from 0.1 to 20 mbar and in particular from 1 to 10 mbar, the temperature of the wall of the evaporation apparatus being from 80 to 160xc2x0 C. and preferably from 110 to 160xc2x0 C. The distillation is preferably carried out with the aid of a mechanically agitated thin-film evaporator or a short-path evaporator and provides pure lactic acid. Optionally, a postconcentration step can be carried out between the second and the third step. This postconcentration is likewise preferably carried out by means of a mechanically agitated thin-film evaporator or a short-path evaporator at a pressure of from 10 to 500 mbar, preferably from 50 to 250 mbar, and at a temperature of from 50 to 150xc2x0 C., preferably from 80 to 120xc2x0 C. It is claimed that in the course of said postconcentration the concentration of the solution as obtained from the first concentration step (step two) can be increased to 100% of lactic acid. A drawback of this method is that in the course of the first concentration step (step two) a poor separation is effected between the concentrated lactic acid-containing solution and the mainly water-containing fraction, so that the mainly water-containing fraction contains a considerable amount of lactic acid and impurities such as oligomers of lactic acid, which is deleterious to the efficiency of the method. Moreover, said water-containing fraction must be purified before it can be discharged or be recycled into the method.
Further methods of preparing and purifying an organic acid such as lactic acid or citric acid by means of fermentation are described, for example, in WO 95/32301, WO 93/06226 (lactic acid), WO 93/00440 (lactic acid), U.S. Pat. No. 3,944,606 (citric acid), U.S. Pat. No. 4,275,234 (citric acid, lactic acid, oxalic acid and phosphoric acid), U.S. Pat. No. 5,132,456 (lactic acid, fumaric acid, succinic acid, maleic acid, adipic acid, itaconic acid, benzoic acid and salicylic acid), U.S. Pat. No. 5,510,526 (lactic acid), U.S. Pat. No. 5,766,439 (lactic acid), EP A 159,585 (lactic acid), EP A 432,610 (citric acid), and EP A 613,878 (citric acid). It is important to note that the purification of citric acid involves far fewer problems than the purification of lactic acid, as the fermentation is able to give rise to citric acid at a far higher concentration (about 40 percent by weight). In the fermentation to produce lactic acid, a concentration of about 30 percent by weight at most of calcium lactate (corresponding to about 21 per cent by weight of lactic acid) can be achieved.
WO 95/32301 describes a method of continuous preparation of an organic acid or the salt thereof, in the course of which fermentation in a bioreactor produces an aqueous acidic solution which is passed over an anion exchanger. The acid is released from the column by the column being treated with an alkali metal hydroxide.
WO 93/06226 describes a fermentation for producing lactic acid, in which a fermentation product is obtained which mainly contains undissociated lactic acid. The lactic acid is recovered by the fermentation product being brought into contact with a solid polymer containing pyridine groups, the acid being absorbed by the polymer.
WO 93/00440 describes a method of preparing lactic acid esters and lactic acid from a fermentation product, which involves the simultaneous reaction of a strong acid, an alcohol and a concentrated fermentation product which mainly contains lactate, and the removal of an azeotropic mixture of water and alcohol. The esters are purified by means of filtration and distillation.
U.S. Pat. No. 5,132,456 describes a method for recovering a carboxylic acid such as lactic acid from an aqueous composition having a pH which is close to the pKa of the acid, which involves purification of the acid by means of an anion exchanger.
U.S. Pat. No. 5,510,526 describes a method of preparing lactic acid from a lactate-containing feed by means of extraction, which involves the feed being brought into contact, under partial carbon dioxide pressure of at least 50 psi(g) (about 3.5 bar) with a water-immiscible amine which contains at least 18 carbon atoms, so that the acid is taken up in the organic amine phase. The acid is released again by means of back extraction.
U.S. Pat. No. 5,766,439 describes a method of preparing an organic acid such as lactic acid by fermentation, which involves an aqueous solution of the salt of the acid being formed. The salt is converted into an ammonium salt from which the acid is subsequently obtained.
EP A 159,585 describes a method of preparing lactic acid, in the course of whichxe2x80x94after fermentation and acidificationxe2x80x94lactic acid is directly subjected, ie. without a filtration step to remove solids, to an extraction, preferably using an alcohol, in a special extraction column.
The methods described in the prior art of preparing and purifying lactic acid are unsatisfactory, however, since lactic acid is obtained at a relatively low yield. Moreover, the purity of the lactic acid obtained is inadequate for certain applications such as applications in pharmaceutical and cosmetic compositions, industrial applications such as in coating compositions for electronic components (for example chips) and environment-friendly applications (e.g. preparation of biodegradable polylactic acid).
It is therefore an object of the present invention to provide a method by means of which lactic acid can be prepared efficiently and very cleanly. The invention therefore relates to a continuous process for preparing lactic acid, wherein the process comprises the following steps of:
(a) acidifying an aqueous feed [1], which has been obtained by fermentation and mainly contains lactate, with a stream [2] containing inorganic acid to a pH of from 1 to 4, preferably from 1.5 to 3 and especially from 1.7 to 2.3, to form an aqueous stream [3] which mainly consists of lactic acid and a salt,
(b) removing the salt from the aqueous stream [3] which mainly contains lactic acid and salt by means of filtration (or decanting) to form a first aqueous stream [4] which mainly contains lactic acid,
(c) passing the first aqueous stream [4] which mainly contains lactic acid over a column containing activated carbon to form a second aqueous stream [5] which mainly contains lactic acid,
(d) subjecting the second aqueous stream [5] which mainly contains lactic acid to a first extraction step, wherein the second aqueous stream [5] which mainly contains lactic acid is brought into contact with a substantially water-insoluble stream [6] which contains an extractant, to form an organic phase [7] which mainly contains lactic acid and extractant and a first water phase [8] which mainly contains impurities,
(e) subjecting the organic phase [7] which mainly contains lactic acid and extractant to a second extraction step, wherein the organic phase [7] which mainly contains lactic acid and extractant is brought into contact with an aqueous stream [9] to form a water phase [10] which mainly contains lactic acid and an organic phase [11] which mainly contains extractant, wherein the organic phase [11] which mainly contains extractant is recycled to step (d), and
(f) concentrating the water phase [10] which mainly contains lactic acid by means of evaporation of water to form a concentrated lactic acid solution in water [12].
As already stated, the term xe2x80x9ca feed which mainly contains lactatexe2x80x9d relates to a feed which contains about 250 g of lactate ion/l or less, based on the total amount of the feed, at least 90 wt % of the lactate ion being present as free lactate ion and the remaining amount of the lactate ion being present as undissociated acid. It will, however, be evident to those skilled in the art that other lactic acid- and/or lactate-containing aqueous solutions can be purified using the method according to the invention. Thus it will be possible for a solution which mainly contains lactic acid to be purified by carrying out only the steps (c)-(f).